We propose three classical coupling methods (Otto, Kretschmann, and grating configurations) to excite graphene magneto-plasmons (MPs). After MP excitation, extraordinary Kerr rotations are obtained. As a demonstration, we discuss the Otto configuration in detail, but the other two methods have similar mechanisms and are only given limited descriptions. The dispersion relations of the MPs are discussed and found to well match previous predictions. It is found that the MP excitation and critical coupling are directly responsible for the large Kerr rotations. Combining the graphene MPs and critical coupling, much larger Kerr rotations (easily above 20°) away from the cyclotron resonance are achieved, as compared to previously described Faraday rotations (below 10°) using graphene MPs. By optimizing the graphene properties and structure parameters, the Kerr rotations are further enhanced. Moreover, using the grating coupling method, we easily find the largest Kerr rotation over 50° at 3 THz. The results are calculated using the anisotropic scattering-matrix method, and the finite element method is used as a comparison. The calculation methods are discussed in detail as a basis for future studies. These results are helpful not only to better understand graphene MPs but also for their potential terahertz applications.